Vedelate biokütustega töötava kolbmootori toitesüsteem

Abstract

Most commonly, standard biofuels, e.g. bioethanol E85, biodiesel, biogas, are used. The use of nonstandard biofuels in engines is not widely spread since their physicochemical properties bring about several issues in the engine as well as in the fuel supply system; nevertheless, their manufacturing is less costly (Küüt, 2013). Some of the issues include the increase in the concentration of harmful components in the exhaust gas, the wearing of the fuel supply system, corrosion, excessive soot in the engine, the coking of the injectors, the injectability, ignition, etc (Courtoy et al., 2009; Ma et al., 2004). To reduce the impact of biofuels on the engine and the fuel supply system, fuel blends are taken into use as the mixing of a biofuel with a standard fuel enables to reduce the impact of biofuels. With regard to the wearing of the fuel supply system, dual-fuel supply systems have been developed as these enable the joint use of biofuels and standard fuels. For the use of nonstandard biofuels, it is rational to use either dual-fuel supply systems or additional fuel supply systems. Engines equipped with an additional fuel supply system run on two fuels whereas the standard fuel is injected into the engine by one fuel supply system and the biofuel by the other fuel supply system (examples in literature include Masahiro, 2003; Cipo & Bhana., 2009). A relevant feature in case of the given solution lies in the fact that the air-fuel mixture is formed of two fuels. It is rational to use dual-fuel supply systems in case of spark ignition engines and additional fuel supply systems in case of compression ignition engines. The use of biofuels presents a challenge regarding the concentration of components in the exhaust gas of the engine. For example, when using diesel fuel and bioethanol in a compression ignition engine, the combustion temperature reduces and the ignition delay increases which condition the drop in the combustion pressure and the increase in the concentration of HC in the exhaust gas. To improve the combustion process, one of the possibilities would be to reduce the size of fuel droplets in the air-fuel mixture formed of bioethanol to accelerate the evaporation of bioethanol. This would enable to decrease the combustion time of the airfuel mixture conditioned by the ignition delay. This particular solution enables the use of bioethanol in a compression ignition engine without altering the settings of the engine, e.g. injection timing (Abu-Qudais et al., 2000; Kowalewicz & Pajączek, 2003; Paper I). The aim of the doctoral thesis was to develop a fuel supply system that would enable the dosing of different biofuels into the cylinders of spark and compression ignition engines while ensuring the formation of small fuel droplets in the air-fuel mixture as well as the resistance of the fuel supply system to the physicochemical properties of biofuels. Based on the aim, the first task was to analyse the suitability of existing fuel supply systems for using biofuels in spark and compression ignition engines. The development of the fuel supply system first required an overview of the combustion process of a compression ignition engine as well as of the impact of biofuels on the exhaust gas emission. The abovementioned overview was comprised based on bioethanol fuel. Tests were carried out with the developed fuel supply system during which the formation of the air-fuel mixture and the size of fuel droplets in the air-fuel mixture were studied. The fuel supply system was developed for use in spark and compression ignition engines. Respective tests were carried out. Engine tests were run using 96.4% bioethanol. Based on test results, the impact of the fuel supply system and bioethanol on the combustion process was analyzed. The overall results of the doctoral thesis are as follows: 1. The existing fuel supply systems are designed for the dosing of one type of fuel into an engine. The fuel supply systems of gasoline are suitable for dosing bioethanol and biomethanol, however, their use brings about the wearing of work surfaces and the creation of corrosion. Fuel supply systems designed for the use of diesel fuel are suitable for dosing vegetable oils into the engine (Fendt, Valtra). The main issues in case of vegetable oils include their ignition, combustion, flowability, etc. The formation of the air-fuel mixture is relevant as the physicochemical properties of biofuels differ from those of standard fuels, thus, the quality formation of the air-fuel mixture ensures an effective evaporation and combustion of the air-fuel mixture in the cylinder. 2. When bioethanol is used in a compression ignition engine as an additional fuel, the exhaust gas emission of engines equipped with common rail increases (HC, CO, CO2). In case of engines equipped with a mechanical fuel supply apparatus, the exhaust gas emission mostly decreases (except for HC). To reduce the exhaust gas emission, it is relevant to ensure the high quality air-fuel mixture in the cylinder and the stability of the combustion process. The combustion pressure p of the combustion process can be increased by reducing the size of fuel droplets in the air-fuel mixture. To calculate the combus103 tion pressure p, a model was drawn which enables to calculate the combustion pressure according to the size of droplets in the air-fuel mixture. It is relevant that in case of big fuel droplets (D32 = 100 μm and larger), the increase in the combustion pressure in the cylinder is slow. To be more precise, in case the size range of fuel droplets is D32 = 500…100 μm, the combustion pressure p doubles, respectively to the reduction in the size of fuel droplets. With regard to the droplet size range of D32 = 100…20 μm, the increase in the combustion pressure p is substantially quicker as compared with big fuel droplets. The combustion pressure p increases in case of fuel droplet size range of D32 = 100…20 μm by approximately a twice and half times. In case of fuel droplet size range of D32 = 20…5 μm, the combustion pressure p roughly doubles. Based on the abovementioned, it can be deduced that as the diameter of a fuel droplet is reduced, the combustion pressure p in the cylinder increases. To ensure the stabile operating of the engine and controlled combustion in the cylinder, the recommended size of fuel droplets in the air-fuel mixture is D32 = 15…100 μm. If the fuel droplets are too small, a detonative combustion may occur, which might damage the engine. The abovementioned calculation model also includes several other parameters that depend on the construction of the engine, environmental conditions, etc. Therefore, the combustion pressure calculated by the model may vary in different conditions from 7 – 60%. 3. During the course of the doctoral thesis, two novel fuel supply system solutions were developed (patents EE05665B1 and EE05693B1). Patent EE05665B1 describes a method of air-fuel mixture formation and a fuel supply system operating based on this particular method. The fuel supply system enables to form high quality air-fuel mixture of different fuels and it can be used as an additional or main fuel supply system. Patent EE05693B1 describes an additional fuel supply system which enables to dose liquid biofuels into an engine. This system can only be used as an additional fuel supply system. During the course of work, the solution presented in the patent document EE05665B1 was chosen and it was developed with regard to achieving the aims of the thesis. As a result, a novel fuel supply system was developed, which is more precisely described in the patent application document P201200024 (Patent III) still in process. 4. In case of the air-fuel mixture formed with the developed fuel supply system, the size of fuel droplets in the air-fuel mixture was studied. The results indicated that the size of fuel droplets is mostly affected by the working parameters of the fuel supply system (injection pres104 sure, the distance between injectors) and the viscosity of fuel. As the injection pressure increased, the size of fuel droplets generally reduced, however, the optimal injection pressure in the particular system was determined to be pa = 2 bar. During tests, it became evident that while using diesel fuel, the size of fuel droplets in the air-fuel mixture formed by the developed fuel supply system was smaller than in the air-fuel mixture formed by the mechanical diesel fuel supply apparatus. In case of the developed fuel supply system, the average size of fuel droplets during bioethanol injection was D32 = 22.5 μm. In comparison to the common fuel supply systems developed for dosing gasoline (in-direct injection systems), the size of fuel droplets formed by the pulveriser fuel supply system is approximately four times smaller (the size of bioethanol fuel droplets in a regular PFI injector is D32 ≈ 80 μm). 5. Using the novel fuel supply system as an additional fuel supply system of a compression ignition engine and dosing 96.4% bioethanol into the engine as an additional fuel, the following conclusions were arrived at: 5.1. The construction and control devices of the fuel supply system are suitable for use on a compression ignition engine. 5.2. During tests, it became evident that using bioethanol as a diesel engine fuel is rational at elevated rotational speeds on engine idle mode (the nominal rotational speed of the engine) or on engine load. The heat release from the process is too small on engine idle mode to ensure the quality combustion of bioethanol fuel. 5.3. The stability of the combustion process, the increase in the heat release rate and the intensified heat release are ensured when an airfuel mixture of small fuel droplet size (bioethanol proportion of up to 25%) is used at elevated rotational speeds on idle mode in a compression ignition engine. When the proportion of bioethanol in the airfuel mixture is increased over 25%, the intensity of heat release and the combustion pressure decrease whereas the proportion of gross heat release in the combustion process increases. 5.4. At elevated rotational speeds of the crankshaft in a compression ignition engine (an engine equipped with a mechanical fuel supply apparatus), generally, the proportion of HC in the exhaust gas increases. The proportion of HC increased in case of air-fuel mixtures with different bioethanol fuel proportions. This may have been caused by the creation of water vapour during bioethanol combustion which hinders the complete combustion of diesel fuel. This particular issue requires complementary study in further research. 6. When using the novel fuel supply system in a spark ignition engine, the following conclusions were arrived at: 6.1. When using the pulveriser fuel supply system, fuel consumption decreased in comparison to the original fuel supply system of the test engine as follows: by ~6% in case of gasoline and by ~3% in case of bioethanol. During tests, it became evident that as engine load increased, the bioethanol consumption of the engine increased rapidly in the pulveriser fuel supply system. This was conditioned by firstly, the construction of the fuel supply system, which did not enable the sufficient air flow into the engine, and secondly, ignition timing, which conditioned the quicker combustion of the air-fuel mixture. As a result, the maximum value of combustion pressure was achieved at top dead centre of the engine or a few crank angle degrees after it, which may have caused the countermining of the combustion pressure to the upward movement of the piston. 6.2. When using an air-fuel mixture of small fuel droplet size, the combustion pressure and heat release rate increased in the engine. In further research, it is necessary to adjust the ignition angle so it would be suitable for the engine since this would ensure the maximum of combustion pressure after the top dead centre of the piston. During the use of the pulveriser fuel supply system, the combustion pressure and heat release rate increased. In further research, it is necessary adjust the ignition angle to be suitable for the engine so that the maximum of combustion pressure after the top dead centre of the piston would be ensured. 7. During tests with the developed fuel supply system, it became evident that its use enabled to reduce fuel consumption in the engine and ensure the effective combustion of the air-fuel mixture in the engine. An increase in the combustion pressure is conditioned by several influencing factors, e.g. the size of fuel droplets, however, combustion pressure might be affected by the length of the intake manifold, the homogeneity of the air-fuel mixture and the construction of the fuel supply system. The particular thesis outlines the positive effect of the fuel supply system on the combustion process of the engine; nevertheless, it is necessary to further research which of the abovementioned influencing factors impacts the efficiency of the particular system more specifically. 8. In case of the given solution, it is first necessary to solve several constructional issues of the system before arriving at final conclusions about the effectiveness of the fuel supply system. However, these issues can be solved during the further design process of the product.Biokütuste kasutamine mootorikütusena on üha kasvav trend. Peamiselt on kasutusel standardbiokütused, näiteks bioetanool E85, biodiislikütus, biogaas. Mittestandardsete biokütuste kasutamine mootorikütustena ei ole laialdaselt levinud, sest nende füüsikalis-keemilised omadused põhjustavad mitmeid probleeme mootorites ning toitesüsteemides, samas on nende tootmine odavam (Küüt, et al. 2013). Probleemideks on ka mootorite heitgaasides sisalduvate ühendite kontsentratsiooni suurenemine, toitesüsteemide kulumine, korrosioon, mootori tahmumine, pihustite koksistumine, pihustatavus, süttivus jne (Courtoy et al., 2009; Ma et al., 2004). Vähendamaks biokütuste mõju mootorile ning toitesüsteemile, on kasutusele võetud kütusesegud, kus biokütuse segamine tavakütusega võimaldab vähendada biokütuste mõju mootorile ning toitesüsteemile. Käsitledes ainult toitesüsteemi kulumist, on väljatöötatud ka kahesüsteemsed toitesüsteemid, mis võimaldavad biokütuste ja standardkütuste koos kasutamist. Mittestandardsete biokütuste kasutamiseks on otstarbekas kasutada kahesüsteemseid toitesüsteeme või lisatoitesüsteeme. Lisatoitesüsteemiga varustatud mootorid töötavad kahe kütusega, kus standardkütus pihustatakse mootorisse ühe toitesüsteemi abil ning biokütus teise toitesüsteemi abil (näiteid allikates (Masahiro, 2003; Cipo & Bhana., 2009)). Oluliseks tunnusteks antud lahenduse puhul on, et küttesegu moodustatakse kahest kütusest. Sädesüütega mootorite puhul on otstarbekas kasutada kahesüsteemseid toitesüsteeme, survesüütega mootorite puhul lisatoitesüsteeme. Biokütuste kasutamisel on probleemiks mootori heitgaasides sisalduvate ühendite osakaal, sest kasutades näiteks survesüütega mootoris diislikütust ning bioetanooli, väheneb põlemise temperatuur mis tingib põlemisrõhu languse ning HC osakaalu suurenemise heitgaasides. Põlemisprotsessi parendamiseks on üheks võimaluseks vähendada bioetanoolist moodustatud küttesegus sisalduvate piiskade suurust, et kiirendada bioetanooli aurustumist. See võimaldab vähendada süüteviivisest tingitud küttesegu põlemise aega. Antud lahendus võimaldab kasutada bioetanooli survesüütega mootori kütusena, muutmata mootori seadistusi, näiteks pritsenurka (Abu-Qudais et al., 2000; Kowalewicz & Pajączek, 2003; Paper I). Doktoritöö eesmärgiks oli välja töötada toitesüsteem, mis võimaldaks doseerida erinevaid biokütuseid säde- ja survesüütega mootori silindrisse, 112 tagades väikeste kütusepiiskade moodustumise küttesegus ja toitesüsteemi vastupidavuse biokütuste füüsikalis-keemilistele omadustele. Tulenevalt eesmärgist oli esimeseks ülesandeks analüüsida olemasolevate toitesüsteemide sobivust biokütuste kasutamiseks säde- ja survesüütega mootorites. Toitesüsteemi väljatöötamiseks anti ülevaade survesüütega mootori põlemisprotsessist ning biokütuse mõjust heitgaaside emisioonile. Eelmainitud ülevaade koostati bioetanoolkütuse näitel. Väljatöötatud toitesüsteemiga on läbiviidud katsetused, kus on uuritud selle küttesegu moodustamist ning küttesegus sisalduvate kütusepiikade suurust. Toitesüsteem on arendatud töötama säde- ja survesüütega mootoritel. Mootorikatsetused on läbiviidud 96.4% bioetanooliga. Katsetustel saadud andmete põhjal on analüüsitud toitesüsteemi ja bioetanooli mõju mootorite põlemisprotsessile. Doktoritöö tulemused on kokkuvõtvalt järgmised: 1. Olemasolevad toitesüsteemid on ettenähtud ühte tüüpi kütuse doseerimiseks mootorisse. Mootoribensiini toitesüsteemid sobivad bioetanooli ja –metanooli doseerimiseks kuid nende kasutamise probleemsus seisneb peamiselt tööpindade kulumises ning korrosiooni tekkimises. Toitesüsteemid, mis on ettenähtud diislikütuse kasutamiseks, sobivad taimsete õlide doseerimiseks mootorisse (Fendt, Valtra). Peamisteks probleemideks taimsete õlide puhul on nende süttimine ja põlemine, voolavus jne. Oluline on küttesegu moodustumine, sest biokütuste füüsikalis-keemilised omadused erinevad standardkütuste omadest, mistõttu tagab kvaliteetne küttsegu moodustumine efektiivse küttsegu aurustumise ja põlemise silindris. 2. Bioetanooli kasutamisel survesüütega mootori lisakütusena, suureneb ühisanumaga varustatud mootorite heitgaaside emisioon (HC, CO, CO2 suureneb). Mehhaanilise toiteaparatuuriga varustatud mootorite puhul heitgaaside emisioon peamiselt väheneb (va. HC). Heitgaaside emisiooni vähendamiseks on oluline tagada kvaliteetne küttesegu silindris ning põlemisprotsessi stabiilsus. Põlemisprotsessi põlemisrõhku p saab suurendada kütusepiiskade suuruse vähendamisega küttesegus. Põlemisrõhu p arvutamiseks on koostatud mudel, mis võimaldab arvutada põlemisrõhku p vastavalt piisa suurusele küttesegus. Oluline on, et suurte kütusepiiskade puhul, D32 = 100 μm ja suuremad, on põlemisrõhu kasv silindris aeglane. See tähendab, et näiteks kütusepiiskade suurusulatuse D32 = 500…100 μm puhul, suureneb põlemisrõhk p kaks korda, vastavalt piisa suuruse vähenemisele. Vaadeldes piiskade suurusulatust D32 = 100…20 μm, on põlemisrõhu p kasv 113 oluliselt kiirem võrreldes suurte kütusepiiskadega. Põlemisrõhk p kasvab kütusepiiskade suurusulatuse D32 = 100…20 μm puhul ligikaudu kaks ja pool korda. Kütusepiiskade suurusulatuses D32 = 20…5 μm suureneb põlemisrõhk p ligikaudu kaks korda. Eeltoodust saab järeldada, et kütusepiisa läbimõõdu vähenemisel suureneb põlemisrõhk p silindris. Tagamaks mootori stabiilne töö ning kontrollitud põlemine silindris, on soovitatav kütusepiiskade suurus küttesegus D32 = 15…100 μm. Liialt väikeste kütusepiiskade puhul võib tekkida detoneeriv põlemine, mis võib kahjustada mootorit. Eelmainitud arvutusmudel hõlmab ka mitmeid teisi parameetreid, mis sõltuvad mootori konstruktsioonist, keskkonnatingimustest jne. Seepärast võib mudeliga arvutatav põlemisrõhk muutuda erinevate tingimuste korral 7 – 60%. 3. Doktoritöö käigus on välja töötatud kaks uudset toitesüsteemi lahendust (patent EE05665B1 ja EE05693B1). Patent EE05665B1 kirjeldab küttesegu moodustamise meetodit ja antud meetodiga töötavat toitesüsteemi. Toitesüsteem võimaldab moodustada kvaliteetse küttesegu erinevatest kütustest ning seda saab kasutada nii lisa- kui põhitoitesüsteemina. Patent EE05693B1 kirjeldab lisatoitesüsteemi, mis võimaldab doseerida vedelaid biokütuseid mootorisse. Süsteemi saab kasutada ainult lisatoitesüsteemina. Töö käigus valiti patendidokumendis EE05665B1toodud lahendus ning arendati seda vastavalt töös tood eesmärkide täitmiseks. Tulemuseks töötati välja uudne toitesüsteemi lahendus, mis on kirjeldatud täpsemalt menetluses olevas patenditaotluses P201200024. 4. Välja töötatud toitesüsteemiga moodustatud küttesegu puhul on uuritud kütusepiiskade suurust küttesegus. Tulemustest selgus, et kütusepiiskade suurust mõjutavad eelkõige toitesüsteemi tööparameetrid (pihustusrõhk, pihustite vaheline kaugus) ning kütuse viskoossus. Pihustusrõhu kasvades vähenes üldjuhul kütusepiiskade suurus, kuid optimaalseks pihustusrõhuks antud süsteemis määrati pa = 2 bar. Katsetustes selgus, et väljatöötatud toitesüsteemi moodustatud küttesegus sisalduvate kütusepiiskade suurus on väiksem, kui tavapärase mehhaanilise diiseltoiteaparatuuriga moodustatud küttesegu. Bioetanooli pihustamisel oli välja töötatud toitesüsteemi puhul kütusepiiskade keskmiseks suuruseks D32 = 22.5 μm. Võrreldes tavapäraste mootoribensiini doseerimiseks väljatöötatud toitesüsteemidega on piiskade suurus ligikaudu neli korda väiksem (bioetanooli kütusepiiskade suurus tavapärase mootoribensiini pihustiga on D32 ≈ 80 μm). 114 5. Kasutades uudset toitesüsteemi survesüütega mootori lisatoitesüsteemina ning doseerides mootorisse lisakütusena 96.4% bioetanooli, saadi järgmised tulemused: 5.1. Toitesüsteemi konstruktsioon ja juhtseadmed sobivad survesüütega mootoril kasutamiseks. 5.2. Katsetustel ilmnes, et bioetanooli kasutamine diiselmootori kütusena on otstarbekas mootori kõrgendatud tühikäigu pöörlemissagedustel (mootori nimipöörlemissagedusel) või mootori koormamisel. Mootori töötamisel tühikäigul, on protsessist eralduv energia liialt väike, et tagada bioetanoolkütuse kvaliteetne põlemine. 5.3. Survesüütega mootori kõrgendatud tühikäigupööretel, peenefraktsioonilise bioetanooli, kuni 25% osakaaluga küttesegu kasutamisel, tagatakse põlemisprotsessi stabiilsus, põlemisrõ